This invention relates to a multiplex speech transmission system, and more particularly to a digital multiplex speech transmission system.
With recent increase in the amount of information to be transmitted, a pulse code modulation system, using digital mulitplex speech transmission systems and capable of efficiently utilizing existing lines have become of interest and commercially used. According to the pulse code modulation system a series of codes is used which are produced by sampling a speech waveform with a frequency of about twice the frequency band width occupied by the speech waveform, and quantizing at a suitable pitch the amplitude of the speech waveform at respective sampling points. With a pulse code modulation system utilized in ordinary speech transmission, speech can be transmitted at high quality by using a sampling frequency of approximately 8 KHz and by quantizing the amplitude to form 7 to 8 bits by adding logarithmic non-linear compressing characteristics. One example of such system is described in a paper of the title "Short Distance 24 Channel PCM System" in Electrical Communication Laboratory Announcing Article, Vol. 12, published Oct. 10, 1965 by Nippon Telegraph & Telephone Public Corporation.
When compared with the conventional analogue speech transmission line of a carrier cable system, the pulse code modulation system described in that paper is advantageous in that it can transmit a large quantity of information because speech signals are transmitted over a single analogue transmission line by using a multiplex system. However, the transmission loss of such system is higher than that of an analogue transmission system so that it is necessary to install regenerative repeaters at a spacing shorter than the length of the analogue transmission line. Accordingly, where such pulse code modulation system is substituted for a conventional analogue speech transmission line, as the length of the transmission line increases the ratio of the cost of the repeaters to the cost of entire installation increases, thus increasing the cost of the pulse code modulation system to be more than that of the analogue speech transmission line. For this reason, at present, the application of the digital multiplex speech transmission system is limited to interoffice trunk lines having relatively short lengths.
Considering the rate of information transmission of the pulse code modulation system described above, as from 56 to 64 kilobits per second are required for each speech channel, researches have been made to have more efficient utilization of the transmission line by decreasing the rate of information transmission. For example, so-called vocoder system has been developed in which correlation characteristics contained in the speech waves are utilized, so as to compress the signal information rate thereby decreasing the rate of information transmission. Such vocoder system is described on pages 720-734 of No. 5, Vol. 54, 1966 of "Proceedings of the I.E.E.E." This system can decrease the rate of information transmission from a fraction to one-tenth of that of the pulse code modulation system described above. With this system, however, a large number of component parts are required for fabricating filters corresponding to digital speech signals, thus increasing the cost of the entire installation. Thus, this system too is not satisfactory for the transmission of information.